"Nothing is impossible" - This means that anything is possible. However, just because something is possible does not mean that it is easy, or that it should be done at all. If you use this statement, you are probably referring to something which is very difficult to accomplish.
"Impossible is nothing" - This means that anything which you consider "impossible" is easy to the speaker. Normally, the speaker would be referring to one specific thing which has already been mentioned. He/she is saying that he/she can accomplish this thing with little effort.